maint: include ctype.h selectively

Include ctype.h only in files that need it.  Many of its uses
are incorrect, as they assume single-byte locales.  The idea is
to remove the incorrect uses later, when there is time.
* src/chroot.c, src/csplit.c, src/dd.c, src/digest.c, src/dircolors.c:
* src/expand-common.c, src/expand.c, src/fmt.c, src/fold.c, src/ls.c:
* src/od.c, src/pinky.c, src/pr.c, src/ptx.c, src/seq.c:
* src/set-fields.c, src/split.c, src/stdbuf.c, src/test.c:
* src/tr.c, src/truncate.c, src/unexpand.c, src/wc.c:
Include ctype.h.
* src/system.h: Do not include ctype.h.

include ctype.h.o

src/split.c

@@ -21,6 +21,7 @@
    * support --suppress-matched as in csplit.  */
 #include <config.h>
 
+#include <ctype.h>
 #include <stdckdint.h>
 #include <stdio.h>
 #include <getopt.h>